This invention relates to an improved method for producing highly viscous materials from subterranean formations. In another aspect, this invention relates to an improved method for preheating viscous materials contained in a subterranean formation whereby the materials can be produced from such subterranean formation. In still another aspect, this invention relates to an improved method for recovering viscous materials from a subterranean formation by selectively preheating a portion of the formation and, thereafter, recovering substantially all of the viscous materials from said formation.
For many years, it has been known that large deposits of very viscous materials, such as tar, heavy crude oil, and the like, are present in subterranean formations. For example, the presence of vast quantities of tar sands has been discussed in detail in Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, Anthony Standen, Editor, Interscience Publishers, New York, 1969, Vol. 19, pgs. 682-732. Such materials are known to have viscosities greater than at least 5,000 centipoises and the majority of such materials have a viscosity in the range of 500,000 to 5,000,000 centipoises at 50.degree. F.
With the realization that conventional energy supplies, such as low viscosity crude oil, natural gas and the like, are being depleted at a rapid rate, as well as certain poetical and economic considerations that have increased the cost of conventional energy sources, there is a considerable amount of interest in recovering and utilizing such viscous materials.
As a result of the increased interest in recovering such viscous materials, many techniques have been suggested for such recovery. One technique has been a suggestion of mining deposits containing such viscous materials and, thereafter, separating the viscous materials, such as tar, heavy crude oil, and the like, on the surface. Unfortunately, such a suggestion has not met with widespread use because many of the subterranean formations are very deep within the ground, thereby adding to the cost of the recovery of such materials. Additionally, it is necessary to handle large volumes of the mined material to extract, or remove, the viscous material from sand, clays, and the like. Therefore, it is evident that conventional mining of materials that contain the viscous tar or oil components is extremely expensive and uneconomical.
The prior art discloses several different techniques for an in situ removal of viscous material such as tar and oil components from subterranean formations. Such techniques include a variety of flooding techniques, such as fire floods, exotic emulsion steam drives, atomic explosions and the like. Despite the large number of techniques that have been tried, most of them have met with little success because of the expense and the low percent of recovery of the viscous materials within the subterranean formations.
Recently, other techniques for recovering viscous materials from subterranean formations, such as the recovery of tar from subterranean tar sand formations have been suggested and utilized with some limited success. For example, a relatively recently disclosed technique involves the use of electrical current to preheat a subterranean formation to elevate the temperature of the formation to a point where the viscosity of the tar or oil components is lowered. By preheating the formation, the viscosity can be decreased to a level where the tar or oil components will flow and to a point where they can be driven from the subterranean formation by means of injection and production wells that have been drilled into the subterranean formation. In such procedures, the injection and production wells are adapted to receive an electrode means and electrical current can be passed through the subterranean formation between the injection wells and the production wells. This type of procedure for recovering viscous materials from subterranean formations is discussed in such patents as U.S. Pat. Nos. 3,848,671, issued Nov. 19, 1974; 3,948,319, issued Apr. 6, 1976; 3,642,066, issued Feb. 15, 1972; and 3,862,662, issued Jan. 28, 1975.
The above-mentioned prior art methods for including electrode means in production and injection wells to preheat the subterranean formation and, thereby, lower the viscosity of the viscous material contained therein have several drawbacks. For example, the production and injection wells are normally drilled on spacings whereby the electrodes are widely separated and spaced apart. Thus, when electrical currents are passed through the formation to preheat the formation, a tremendous amount of electrical energy is needed. This tremendous amount of electrical energy is needed because the prior art methods provide for an essential preheating of the entire formation before the driving fluid is injected. Additionally, there is a considerable amount of energy loss due to the fact that the electrical currents do not necessarily flow in straight paths from one electrode to another. Many times, the overburden or underburden above and below the formation containing the viscous material will have a lower electrical resistance than the formation, itself. Thus, the electrical current will enter the overburden or underburden and will travel between the electrodes. Of course, when the electrical current travels through the overburden or underburden, little or none of the desired heating of the viscous material within the subterranean formation is obtained.
Prior art techniques for electrically preheating viscous material contained within subterranean formations also have resulted in other problems because of high current flows in localized areas around the electrodes. It has been found that the areas generally surrounding the electrodes in the prior art methods become overheated and, in some instances, the electrodes, themselves, will melt or be burned away, due to this excessive heat.
Probably, one of the most difficult problems to cope with in the conventional, electrical preheating of subterranean formations is in the area of ultimate recovery of the tar or viscous oil components from the subterranean formation. Specifically, conventional electrical preheating of the subterranean formations will take place in a generally straight line between the injection and production wells. Then, when a driving fluid is injected into the formation or when a fire flood is started in the formation, the tar or viscous oil components laying along this generally straight line between the injection and production wells will be removed from the formation first. The tar or viscous oil materials located away from this straight line between the injection and production wells is extremely difficult to remove and, in many cases, is impossible to remove.
In addition to the foregoing problems that are experienced with conventional methods for electrically preheating subterranean formations, it is also well known that such conventional techniques require an unusually long period of time to properly preheat the formation for ultimate recovery of the viscous material therefrom. It is not unusual for preheating to be carried out for a matter of months or years in order to preheat a subterranean formation to a point where the viscosity of the materials contained therein is reduced to a level where production can begin using such known techniques. Such prolonged preheating periods make such conventional methods unattractive from an economic standpoint for the commercial production of viscous materials.
In view of the foregoing problems and deficiencies of the prior art methods for recovering viscous materials from subterranean formations, it is, of course, highly desirable to develop improved techniques for such recovery.